The brushless motor has been widely used in office automation devices and audio-video devices. Among various brushless motors, a polygon-mirror-scanner-motor employed in laser copying machines, a spindle-motor employed in magnetic-disc-driving-devices are directed to the higher-rotating-speed. The rotating speed of the polygon-mirror-scanner-motor is over 20,000 rpm, and that of the spindle-motor is as high as 12,000 rpm because a memory capacity has been increased and the higher rotating speed has been required.
The higher rotating speed entails the greater vibrations due to powering a motor coil and cogging, and these increased vibrations involve other problems. Regarding noises of the motor, the higher rotating speed induces the motor to rotate in an imbalance manner, which produces additional vibrations. Further, the higher rotating speed increases motor-loss thereby boosting the power consumption. The major loss in power consumption comprises windage loss, axial loss, iron loss and the like. The iron loss among others has two components, i.e. hysteresis loss and eddy-current-loss. In general, the hysteresis loss is proportionate to a number of rotations (more specifically, a current-frequency in the motor coil), and the eddy-current-loss is proportionate to a square of the number of rotations (more specifically, the current-frequency in the motor coil). The eddy-current-loss is thus increased at the greater number of rotations, so that the iron loss takes a greater part of the entire loss.
Smooth rotating is required for the motor to reduce vibration. To achieve this, it is necessary to lower the cogging and eliminate torque ripples. Skew magnetizing to a rotor magnet or laminating a plurality of core pieces forming a stator core is a regular measures to lower the cogging and eliminate the torque ripples.
On the other hand, the following two methods are employed to reduce iron loss:
1. Decreasing the iron loss of the stator core per se by replacing silicon-steel-plates which are laminated to the stator core with the plates having less iron loss, or by annealing the stator core; or PA1 2. Decreasing a number of magnetized polarities thereby lowering the frequency of current running through the motor coil. PA1 .theta.2=skew angle; PA1 2n=a number of magnet poles of the rotor magnet; PA1 3n=a number of slots of the stator core. PA1 (a) hole 251 for the stator core to be fixed; PA1 (b) annular base 252 provided around hole 251; and PA1 (c) teeth 253 protruding from an outer wall of base 252 in a radius direction and being wound by coils 260. End plates 254 protruding in the axial direction are formed on each tip of respective teeth 253. PA1 1. To achieve a method solving the problems applicable with ease to any sizes of motors including a small and a thin sizes. PA1 2. To realize a brushless motor with less torque ripples, cogging and iron loss as well as a better torque constant of motor characteristics. PA1 (a) a rotor magnet including a plurality of magnetic poles; and PA1 (b) a stator core facing the rotor magnet via air gap.
Conventional problems due to skewing provided to the rotor magnet and stator core are discussed hereinafter.
When the skewing is provided to the rotor magnet and stator core, torque ripples and cogging decrease; however, the motor efficiency as well as torque constant is lowered. In recent years, the market strongly demands a smaller device with the less vibrations and the lower noise and yet the device should keep the same torque-constant and the motor-efficiency as those of an original device. To achieve this request, the torque ripples and cogging of the motor per se should be reduced.
Japanese Patent Examined Publication No. 2636108 shown in FIG. 5 already disclosed a method how to reduce cogging by providing skewed magnetization to the rotor magnet.
In this prior art shown in FIG. 5, the following formula is established: EQU (76.degree./n).times.0.8 .ltoreq..theta.2.ltoreq.(76.degree./n).times.1.2
where
The structure embodied by this formula allows the skew angle at magnetizing the rotor-magnet to be set so that the cogging and torque ripples are reduced without lowering the motor efficiency.
This prior art is effective only when a motor has some dimensional room, thus the prior art is difficult to apply to the motor to be smaller and thinner. In particular, when the motor becomes thinner and uses only a small number of core layers or the height of rotor magnet is too low to measure, this skewed magnetization produces no effects. In this case, neither lowering the cogging nor decreasing the torque ripples is expected, and the motor characteristics Kt (the torque constant) is aggravated.
The problems accompanying the conventional method of reducing the iron loss are described hereinafter.
A plurality of core pieces, i.e. silicon steel plates, are replaced with the plates having the less iron loss, or the plates undergo annealing. These are usual methods for decreasing the iron loss; however, these methods incur cost increase, and the annealed material is vulnerable to corrosion. An appropriate surface treatment is thus required.
Another method is disclosed in Japanese Patent Application Non-examined Publication No. H07-31085 shown in FIG. 6. The method is to construct the stator core not by laminating a plurality of core pieces but by unitarily forming the stator core, so that the iron loss is reduced.
FIG. 6A is a cross section of a conventional motor, FIG. 6B is a plan view of a stator core of the conventional motor, and FIG. 6C is a side view thereof.
In FIG. 6A, teeth 253 of stator core 250 are wound by coils 260, and rotor magnet 280 is disposed around stator core 250 via an annular space. Magnet 280 is fixed to depending 274 of the rotor.
In FIGS. 6B and 6C, stator core 250 comprises the following elements:
Stator core 250 comprises base 252 and teeth 253. These elements undergo a press-process and a machining process thereby forming the stator core, and then the stator core undergoes annealing process. This method, i.e. forming the stator core not by laminating a plurality of core pieces but by forming unitarily the elements into the stator core before undergoing the annealing process, can reduce the iron loss.
This method, however, requires a number of processing steps such as press-process, machining-process, and thus produces unstable quality and no advantage of cost. Indeed this method can improve a saturated magnetic flux density; however, it produces only a little effect for reducing the iron loss. This method is difficult to apply to a tall-height motor, and aggravates the motor characteristics Kt if this method is employed.